This invention relates to a method for advantageously melting aluminum scraps in a freshly molten aluminum mass (hereinafter referred to as "molten bath") for thepurpose of reutilization of the aluminum contained in the scraps. The term "aluminum scraps" as used in this specification is meant to embrace both scraps of aluminum and scraps of aluminum alloys.
In the melting of aluminum scraps in the molten bath, it is extremely important from the economic point of view to minimize possible loss of aluminum due to melting.
The melting of aluminum scraps is usually performed in the atmosphere. The temperature of the molten bath is very high, generally exceeding 660.degree. C., and frequently ranging from 720.degree. C. to 780.degree. C. When aluminum scraps are introduced into the molten bath at such high temperatures, they are liable to react with oxygen and nitrogen contained in the ambient air and produce aluminum oxide and aluminum nitride. These products are called "melt slag" and they are responsible for said melting loss. To minimize this melting loss, it is imperative that said reaction should be prevented from occurring, i.e. the aluminum scraps should be caused to sink under the molten bath as quickly as possible. For this purpose, there has actually been employed the liquid-heel process, the liquid-flux process or the method which comprises pressing and immersing scraps into the molten bath.
The liquid-heel process comprises the steps of depositing aluminum scraps on the surface of the molten bath and subsequently forcing these aluminum scraps into the interior of the bath. This forced submersion of aluminum scraps is effected by a manual method or a mechanical method. Specifically, the manual method effects the forced submersion of aluminum scraps by causing the scraps floating on the surface of the molten bath to be pushed down into the interior of molten bath by use of a rabbler which consists of an iron bar and a disc or square of iron plate attached at an angle to the forward end of said iron bar. This operation entails immense labor and is the most physically taxing job involved in any kind of melting operation. The mechanical method generally employs bladed puddlers in the case of iron-hot furnaces or grid puddlers in the case of hearth furnaces of various types. These two methods both aim to prevent aluminum scraps from undergoing oxidation or nitrification by minimizing the duration of exposure of such aluminum scraps to the ambient air while they are floating on the surface of molten bath. These methods, however, are such that they cause the aluminum scraps to undergo oxidation and nitrofication to a considerable extent. Thus, they suffer from the heaviest melting loss of all the types of melting methods.
The liquid-flux process accomplishes desired melting of aluminum scraps by introducing said scraps in a mass of flux melted in advance. A typical flux to be used for this purpose consists of 50 to 70% by weight of NaCl, 25 to 45% by weight of KCl, 3 to 10% by weight of CaF.sub.2, etc. for example. In the case of this composition, the flux is melted and kept at temperatures in the range of from 720.degree. C. to 780.degree. C. when the aluminum scraps are introduced. This method entails very little melting loss because the aluminum scraps are melted immediately after their entry into the molten flux. In this method, however, since the molten flux absorbs oxides and consequently becomes viscous and deteriorated, the melting operation requires 200 to 300 kg of molten flux per ton of aluminum scraps placed therein. The cost of melting by this method, therefore, is quite high.
New clippings and solids of pure aluminum (as defined by the United States NASMI Standards for Non-ferrous Metal Scraps) have large surface areas and small bulk densities for the unit weight and, therefore, are quite liable to undergo oxidation and nitrification upon exposure to the ambient air. To cope with the difficulty, there has been adopted a method for decreasing their apparent surface areas per unit weight by pressing the clippings and solids generally under a pressure of 30 kg/cm.sup.2. This pressing indeed increases their bulk densities whose initial values are usually on the order of from 0.28 to 0.48. Actually, however, they are increased by 1.6 times at most.
An object of the present invention is to provide a method for advantageous melting of aluminum scraps, which method neither requires use of any flux nor necessitates forced submersion of floating scraps into the interior of the molten bath but is applicable generally to all types of aluminum scraps and suffers from less melting loss than any other methods directed to the same purpose heretofore put to actual use.